PROJECT SUMMARY/ABSTRACT Training: My research on obesity has focused primarily on the etiology of pediatric obesity, and the natural next step in my academic career is to design, develop, and implement an evidence-based weight loss intervention for adolescents with overweight/obesity (OW/OB). The proposed K01 career development training plan builds upon my prior experience to focus on three areas that require additional training: (1) build skills in designing and implementing obesity weight loss trials in adolescents, (2) develop expertise in behavioral change theories, including acceptance based-therapy (ABT), and (3) enhance skills in implementation science. Accordingly, we propose intensive topical mentorship, focused coursework, practicums, seminars, and contextual learning through the proposed research. This K01 will provide protected time to receive the necessary training to enhance my expertise in these three areas and will propel me to becoming an independent research scientist focused on the development of effective obesity interventions in youth. Research: The prevalence of OW/OB in U.S. adolescents is 34.5%. The effectiveness of currently available adolescent weight loss interventions is limited because the amount of weight lost is modest and weight regain is common. Thus, there is a critical need to develop effective interventions in adolescents with OW/OB. The proposed K01 attempts to address this need and the objective of this K01 proposal is to develop and test the feasibility, acceptability, and efficacy of an ABT weight loss intervention tailored for adolescents with OW/OB. ABT focuses on self-regulation skills and increasing tolerance of negative or uncomfortable emotions, and research demonstrates that it is a highly effective weight loss strategy in adults. Further, ABT has been successful in treating adolescents with chronic pain, high-risk sexual behavior, anorexia, and depression. Given the success of ABT for treating other medical and behavioral issues in adolescents and the high percent of weight loss observed in adults, ABT could represent an effective weight loss intervention for adolescents. However, to date, no one has studied ABT as an adolescent weight loss tool. Thus, the proposed K01 will develop an ABT intervention tailored for adolescents with OW/OB. The overarching hypothesis of this proposal is that an adolescent ABT intervention will be feasible, acceptable, and will result in lower BMI z-scores and body fat percentage, healthier dietary intakes, increased physical activity, and improved blood pressure and cholesterol relative to an enhanced standard care condition. Summary: Findings from this study will inform an R01 grant application to test the effectiveness of an adolescent ABT weight loss intervention in a fully-powered sample. This K01 will develop my new expertise in conducting adolescent obesity trials, applying behavioral change theories to intervention development, and implementation science. At the end of this award, I will be a leading independent scientist conducting effective weight loss interventions in youth.